Plants that defy evolution: Datura stramonium (Jimson Weed)

Some creationists consider the thorns on plants a problem for evolutionary theory. As I will show, this is based on a flawed understanding of evolutionary theory. But before I proceed, let me present the ‘argument‘ presented by Joseph Alden (uncle of  Gary McGuire)

The ‘argument’

Per your request, I am forwarding two more examples of plant species, which DEFY Darwinian evolution.

Examples :

1. The ” jimson weed “

Scientific name : Datura stramonium. This is a basic plant that grows wild, like most other weeds, namely in various parts of the United States. However, this weed is toxic. Its roots, stems and leaves all contain trace amount of natural poisons, including several alkaloid compounds. And yet, the MOST toxic area of the plant is found in the seed pod. This pod is about the size of a walnut and is covered with prickly thorns. Why all the thorns ? They serve NO practical purpose in the growth of the plant, at any stage in its life cycle. Therefore, why the need for all these thorns, located around a pod of toxic seeds ? Does the plant KNOW it is lethal ?
IMPOSSIBLE. It is a PLANT . It has no brain, no central nervous system, no subjective thought process, etc. Why then, would it have added this feature at some point in time, according to evolutionists, and established for itself all these thorns, that serve no purpose in the growth of the plant ? Its contents, by the way are NOT lethal to other plants; only animals. Therefore, could plants know that animals are looming about ? NO. Again, it is impossible ! They have no ability for any analytical thought process. They also have no sensory perception, like animals, so they would not know of potential danger from its toxic seeds. How and why would it possess this defense mechanism ?

Easy. The plant was CREATED by an intelligent designer from the start. Why you ask ? Simple. There IS a benefit to all those toxic seeds. They have been used for medicinal purposes for years. It’s just that, in mass quantity, if consumed by wild animals or humans, they could be potentially fatal. Thus, Creationists can easily explain, through basic scientific observation, the existence of the jimson weed. Evolutionists hate examples, like the jimson weed, because it DISPROVES the deductive reasoning used to justify evolution.

The scientific explanation

Let’s now see how science explains the origin and evolution of thorns and why there are plants which have thorns protecting the seed pods when the seeds themselves are toxic.

Co-evolution in plants

A good review paper of coevolution of defenses in plants is found in Rausher’s  Co-evolution and plant resistance to natural enemies published in Nature,  vol. 411, Jun 2001. We have to remember that there are a variety of potential selective pressures that may guide the evolution of these defenses, that this is a process of co-evolution and that this is a process involving a variety of predators and threats. As I will show the answer to the “paradox” can be easily found when taking into account these issues. Plants have evolved an arsenal of defenses which include resistance through chemical defensive (for instance alkaloids),  induced defenses, and morphological defenses (trichomes (hairs and thorns), cell walls, etc ). Chemical defenses are typically seen as having evolved from metabolic by-products. In addition to resistance, plants also have developed tolerance as an effective co-evolutionary response. Tolerance reduces or eliminates the detrimental effect of damage or infection to the plant. Plants also deploy avoidance even though the plant is not mobile through for instance cryptic coloration.

The phenotypic interface of coevolution. Fitness of each individual involved in interspecific interactions is determined through reciprocal selection both caused and experienced by the phenotypes that mediate the interaction. This phenotypic interface typically involves performance traits that comprise a variety of behavioral, morphological, and physiological components (from Reciprocal Selection at the Phenotypic Interface of Coevolution)

Second of all, plants have been shown to be ‘jack of all trades’

Overall, our survey provides little evidence that genetic trade-offs between defensive traits significantly constrain the evolution of multiple defenses. To a large extent, plants appear to be jacks-of-all-trades, masters of all: they may successfully produce several types of defense without paying considerable trade-offs. Therefore, the original view that multiple defenses are costly and functionally redundant should be replaced by the idea that environmental variability and positive interactions among defenses can make many combinations of defenses possible and evolutionarily advantageous.

Julia Koricheva, Heli Nykanen, and Ernesto Gianoli Meta-analysis of Trade-offs among Plant Antiherbivore  Defenses: Are Plants Jacks-of-All-Trades, Masters of All? The American Naturalist vol. 163 , no . 4 , April 2004

So let’s see how we can explain the ‘paradox’ of Datura Stramonium, aka the Jimson Weed

Datura Stramonium (Jimson Weed)

First the origin of the common name Jimson Weed can be traced back to 1676

Although jimsonweed is synonymous with several species of Datura, this unusual common name is actually derived from D. stramonium. In 1676 British soldiers stationed in Jamestown, Virginia became intoxicated by D. stramonium when it was inadvertently included in their salads by the regimental cooks. The episode was widely publicized and the plant culprit became known as “Jamestown weed”, and later as jimsonweed.

The seeds, leaves, stem, roots and fruits all contain a variety of chemicals

The leaves, stem, root and fruits of Datura contain a battery of tropane alkaloids, the most potent of which are atropine, hyoscyamine and scopolamine. These alkaloids affect the central nervous system, including nerve cells of the brain and spinal cord which control many direct body functions and the behavior of men and women. They may also affect the autonomic nervous system, which includes the regulation of internal organs, heartbeat, circulation and breathing. One autonomic response of atropine is the dilation of pupils, once considered to be a beautiful and mysterious look in Italian women. In fact, belladonna means “beautiful lady,” so named because sap from the closely related belladonna plant (Atropa belladonna) was used as eye drops to dilate the pupils.

In addition to being used as hallucinogens during religious ceremonies, or as sexual stimulants by prostitutes, or as anesthetics (novocain) or as a drug against motion sickness. One may very well conclude that the ‘designer’ surely had a wide range in applications in mind while creating these drugs…

First of all let’s describe Datura stramonium

©Patrick J. Alexander. Provided by Patrick J. Alexander.

As you can observe, the seed pods are protected by thorns and yet, the seeds themselves are toxic to mammals although cattle is known, under stress, to eat the weeds.

Classification:
Datura stramonium L.

Kingdom	Plantae – Plants
Subkingdom	Tracheobionta – Vascular plants
Superdivision	Spermatophyta – Seed plants
Division	Magnoliophyta – Flowering plants
Class	Magnoliopsida – Dicotyledons
Subclass	Asteridae
Order	Solanales
Family	Solanaceae – Potato family
Genus	Datura L. – jimsonweed
Species	Datura stramonium L. – jimsonweed

The Family Solanaceae contains potatoes, eggplant, tobacco as well as nightshade. Another data point: Jimson Weed like most of its family members have been imported from tropical areas. Note that the toxicity of the Jimson Weed is not limited to the seed pods, but includes the leaves, stem, and other parts of the plant. This is important when discussing the relevance of toxic defenses against herbivore insects.

The hypothesis

So now the evolutionary hypothesis

Co-evolution between plants and their natural enemies is generally believed to have generated much of the Earth’s biological diversity. A process analogous to co-evolution occurs in agricultural systems, in which natural enemies adapt to crop resistance introduced by breeding or genetic engineering. Because of this similarity, the investigation of resistance mechanisms in crops is helping to elucidate the workings of co-evolution in nature, while evolutionary principles, including those derived from investigation of co-evolution in nature, are being applied in the management of resistance in genetically engineered crops.

review article Co-evolution and plant resistance to natural enemies Nature 411, 857-864 (14 June 2001)

Thus, in order to understand the evolution of Datura Stramonium, we have to understand its evolutionary history, the original environment(s) in which this evolution took place, and the selective pressures that existed and the genetic variation. In addition we need to show that there are selectable, inheritable traits.

In other words, ecology and molecular biology all need to work together to establish answers to these questions.

Evolution of mixed strategies

In this study we present a simple optimization model for the evolution of defensive strategies (tolerance and resistance) of plants against their natural enemies. The model specifically evaluates the consequences of introducing variable costs and benefits of tolerance and resistance and nonlinear cost-and-benefit functions for tolerance and resistance. Incorporating these assumptions, the present model of plant defense predicts different evolutionary scenarios, not expected by previous work. Basically, the presence of an adaptive peak corresponding to intermediate levels of allocation to tolerance and resistance can arise when the shape parameter of the cost function is higher than the corresponding of the benefit function. The presence of two alternatives peaks of maximum tolerance and maximum resistance occurs only when benefits of tolerance and resistance interact less than additive. Finally, the presence of one peak of maximum resistance or maximum tolerance depends on the relative values of the magnitude of costs for tolerance and resistance. An important outcome of our model is that under a plausible set of conditions, variable costs of tolerance and resistance can represent an important aspect involved in the maintenance of intermediate levels of tolerance and resistance, and in favoring adaptive divergence in plant defensive strategies among populations. The model offers a framework for future theoretical and empirical work toward understanding spatial variation in levels of allocation to different defensive strategies.

EVOLUTION OF MIXED STRATEGIES OF PLANT DEFENSE ALLOCATION AGAINST NATURAL ENEMIES, Evolution, Volume 58, Issue 8 (August 2004)

As such we come to realize that herbivores included both the grazing types (mammals) as well leaf eating types (herbivorous insects).

Thus we need to explain how alkaloid content (the poisonous part of Jimson Weed) is under selective pressure from insect herbivory and how thorns can be both under selective pressures from insect and grazing mammals.

Insect Herbivory and alkaloid content

The first step is to link alkaloid content to insect herbivory.

Abstract.—Although insect herbivory is frequently assumed to be responsible for the maintenance of plant secondary metabolites such as alkaloids, the assumption is controversial and experimental evidence for this assumption is sparse. We examined natural selection on the two major alkaloids present in the leaves of Datura stramonium and found that both alkaloids came under active selection. We found negative directional selection for scopolamine (natural selection acting to reduce scopolamine levels) and stabilizing selection for hyoscyamine (natural selection acting to maintain an intermediate level of hyoscyamine). We also present evidence that insect herbivores act as the agents of selection on these alkaloids. Finally, we show that there were no trade-offs in resistance to different species of insects.

Irene Shonle, Joy Bergelson (2000) EVOLUTIONARY ECOLOGY OF THE TROPANE ALKALOIDS OF DATURA STRAMONIUM L. (SOLANACEAE) Evolution 54 (3) , 778-788

See also the various references to the above paper

Trichome defense and resistance to herbivorous insects

Then there is Trichome and thorn production. Trichromes are “any protrusion from the epidermis“  and include hairs as well as thorns. For Datura Stramonium, trichromes have been shown to be under selective pressure against insects.

This study assessed the role of leaf trichome density as a component of resistance to herbivores, in six populations of Datura stramonium. Phenotypic selection on plant resistance was estimated for each population. A common garden experiment was carried out to determine if population differences in leaf trichome density are genetically based. Among population differences in leaf trichome density, relative resistance and fitness were found. Leaf trichome density was strongly positively correlated to resistance across populations. In 5 out of 6 populations, trichome density was related to resistance, and positive directional selection on resistance to herbivores was detected in three populations. Differences among populations in mean leaf trichome density in the common garden suggest genetic differentiation for this character in Datura stramonium. The results are considered in the light of the adaptive role of leaf trichomes as a component of defence to herbivores, and variable selection among populations.

Defensive role of leaf trichomes in resistance to herbivorous insects in Datura stramonium, Journal of Evolutionary Biology, Volume 14 Issue 3 Page 424-432, May 2001

Conclusions

Contrary to creationists’ claims, evolutionary theory can in fact very well explain the ‘paradox’ that the Jimson Weed has a poisonous seed pod, protected by thorns, understanding that the complex ecologies involve predation by mammals as well as insects, that alkaloid content is a selectable trait in response to insect predation, that alkaloid content extends across the plant, not just the seed pods and that the plant has a variety of defenses against its predators. In addition trichomes have been shown to be selectable defenses against insects and the evolution of thorns to protect the seed pods from predation by mammals may very well have been a simultaneous evolution with alkaloid defenses. While I have yet to do the research into the following area, it is not difficult to imagine how trichome defenses, initially evolved against insect predation and that thorns evolved as an additional protection of the seed pods against predation by larger animals, over time additional defenses came ‘on line’ including chemical defenses against insects.

Other Claims

Its roots, stems and leaves all contain trace amount of natural poisons, including several alkaloid compounds. And yet, the MOST toxic area of the plant is found in the seed pod.

versus

This paper reports a study of the alkaloid content of different parts of Datura stramoniumL. (Solanaceae) in various stages of its growth and development, and the location, structure and ultrastructure of alkaloid-biosynthesizing cells (idioblasts) in different parts of naturally grown and cultured plant material. TLC, HPLC, and GC/MS methods were used for alkaloid assays. The results showed that alkaloid production starts from the end of the second week after seed germination, increases in different organs up to the tenth week of growth, and then decreases. Leaves and capsules showed the highest alkaloid content in the vegetative and generative stages, respectively. In leaves the alkaloids decreased rapidly in the generative stage. The highest alkaloid content was recorded in vegetative leaves, followed in descending order by vegetative petioles, generative and vegetative stems, generative petioles, generative roots, generative leaves, vegetative roots and mature seeds. The organs as well as calli derived from different leaf parts were examined for the presence of idioblasts by microscopic and cytochemical methods. Idioblasts were present only in semi-hyaline callus originated from leaf base; they were spherical or oval, with a thick cell wall and large central vacuole. These observations should prove helpful in attempts to produce specific alkaloids in naturally grown plants and cell cultures.

In DISTRIBUTION OF ATROPINE AND SCOPOLAMINE IN DIFFERENT ORGANS  AND STAGES OF DEVELOPMENT IN DATURA STRAMONIUML. (SOLANACEAE). STRUCTURE AND ULTRASTRUCTURE  OF BIOSYNTHESIZING CELLS

Other factors

Evolutionary ecology of Datura stramonium: equal plant fitness benefits of growth and resistance against herbivory.

Abstract:
This study evaluated how natural selection act upon two proposed alternatives of defence (growth and resistance) against natural enemies in a common garden experiment using genetic material (full-sibs) from three populations of the annual plant Datura stramonium. Genetic and phenotypic correlations were used to search for a negative association between both alternatives of defence. Finally, the presence/absence of natural enemies was manipulated to evaluate the selective value of growth as a response against herbivory. Results indicated the presence of genetic variation for growth and resistance (1 - relative damage), whereas only population differentiation for resistance was detected. No correlation between growth and resistance was detected either at the phenotypic or the genetic level. Selection analysis revealed the presence of equal fitness benefits of growth and resistance among populations. The presence/absence of natural herbivores revealed that herbivory did not alter the pattern of selection on growth. The results indicate that both strategies of defence can evolve simultaneously within populations of D. stramonium.

and

Abstract:
This study evaluated how natural selection act upon two proposed alternatives of defence (growth and resistance) against natural enemies in a common garden experiment using genetic material (full-sibs) from three populations of the annual plant Datura stramonium. Genetic and phenotypic correlations were used to search for a negative association between both alternatives of defence. Finally, the presence/absence of natural enemies was manipulated to evaluate the selective value of growth as a response against herbivory. Results indicated the presence of genetic variation for growth and resistance (1 - relative damage), whereas only population differentiation for resistance was detected. No correlation between growth and resistance was detected either at the phenotypic or the genetic level. Selection analysis revealed the presence of equal fitness benefits of growth and resistance among populations. The presence/absence of natural herbivores revealed that herbivory did not alter the pattern of selection on growth. The results indicate that both strategies of defence can evolve simultaneously within populations of D. stramonium.

VALVERDE, P. L. ; FORNONI, J. ; NUNEZ-FARFAN, J. Evolutionary ecology of Datura stramonium: equal plant fitness benefits of growth and resistance against herbivory. Journal of Evolutionary Biology. 16(1):127-137, January 2003.

Now we look at

Juan Núñez-Farfán, Juan Fornoni, Pedro Luis Valverde The Evolution of Resistance and Tolerance to Herbivores Annual Review of Ecology, Evolution, and Systematics, December 2007, Vol. 38, Pages 541-566

Tolerance and resistance are two different plant defense strategies against herbivores. Empirical evidence in natural populations reveals that individual plants allocate resources simultaneously to both strategies, thus plants exhibit a mixed pattern of defense. In this review we examine the conditions that promote the evolutionary stability of mixed defense strategies in the light of available empirical and theoretical evidence. Given that plant tolerance and resistance are heritable and subject to environmentally dependent selection and genetic constraints, the joint evolution of tolerance and resistance is analyzed, with consideration of multiple species interactions and the plant mating system. The existence of mixed defense strategies in plants makes it necessary to re-explore the coevolutionary process between plants and herbivores, which centered historically on resistance as the only defensive mechanism. In addition, we recognize briefly the potential use of plant tolerance for pest management. Finally, we highlight unresolved issues for future development in this field of evolutionary ecology.

Update 05-22-2008